Neonicotinoid insecticides are the most widely used insecticide class in the world. Pervasively used as seed-treatments for major row crops (e.g., corn, soybeans), neonicotinoids are applied across vast agricultural regions; however these insecticides are persistent, highly water soluble, and are shown to be transported via surface run-off and wind deposition. They continue to receive increased scrutiny due to potential toxicity and sub-lethal effects on non-target organisms (e.g., honeybees). It is unclear if neonicotinoid use reduces native pollinator abundance, diversity and/or alters ecosystem services (e.g., pollination) within agroecosystems. Numerous wild bee species nest in the ground and forage on a range of flowering plants near agricultural fields. We focused on two main questions: (1) What are current neonicotinoid levels in Missouri soils and margin plant communities? (2) How do these levels affect native bee populations in fields and field margins? In 2016, we sampled 24 fields (seed-treated, n = 16, untreated controls, n = 8) on four conservation areas in mid-Missouri from pre-seeding to harvest. We collected soils from agricultural fields and their margins, sampled herbaceous and woody flowering species in field margins, and collected a variety of native pollinators including bees and butterflies over time in fields and field margins.
Our data indicate that neonicotinoids persist and accumulate over time in fields and margins. Both field and margin soils had detectable neonicotinoid residues in all sampling periods analyzed to date (pre-seeding to mid-growing) with peak concentrations detected in the growing period (fields: 47.9 µg/kg; margins: 41.7 µg/kg ). Field-collected plant samples are currently undergoing analysis. Overall, we collected ~2,450 individual native bees representing 72 species. We expected that bee abundance would be lower in treated fields and their margins compared to untreated controls. Our preliminary models indicate that neonicotinoid concentrations in field soils did not affect bee abundance in fields whereas crop type did (P= 0.002). In contrast, abundance was significantly lower (P= <0.001) in field margins where there was an increase in neonicotinoid concentrations in soils. Overall, the size of the margin area (ha) was positively associated with increased bee abundance (P= <0.001). As the majority of native bees are active during the growing period when insecticide concentrations are highest, this could have implications for pollinator conservation. Regardless, our preliminary findings suggest that native bee abundance may be lower in areas of high neonicotinoid exposure.